Most aluminum is produced by the Hall-Heroult process which involves the electrolysis of alumina in a molten cryolite bath using carbon electrodes. The carbon anodes are consumed by the anodic oxidation process with the formation of CO.sub.2 /CO and their life-time is very short, typically about 2 to 3 weeks for the pre-baked type of anode. They also add impurities to the bath. The cathodes, which are also made of carbon but have a longer life-time of more than two years, are covered with a cathodic layer of molten aluminum which has to be maintained very thick in order not to expose the carbon to the bath because the carbon is not wettable by molten aluminum. This high inventory of aluminum in a cell leads to the drawback that the electromagnetic forces produce waves and ripples in the molten aluminum which necessitates a large interelectrode gap and a corresponding high cell voltage.
Many materials and design expedients have been suggested and tried with a view to improving the performance of electrolysis, but so far the results have not been successful. In particular, there have been numerous suggestions for aluminum-wettable cathode materials such as the refractory borides, but these materials are expensive, difficult to manufacture, and difficult to fix as a cell-lining material or to coat them on less expensive substrates. Various composite materials have also been suggested for this purpose (see for example U.S. Pat. Nos. 2,480,475, 3,328,280, 3,408,312, 3,459,515 and 3,661,436) but none of these materials have proven to be acceptable.
Recently, European patent application No. 82/200802.5, published on Feb. 16, 1983 under No. 0072043, has proposed composite materials of aluminum and an aluminum oxycompound, typically alumina, and optionally with additives such as borides, nitrides and carbides, which show great promise for those components of aluminum production cells which in use are normally covered with molten aluminum, including current-carrying components such as a cathode or cathode current feeder, part of a bipolar electrode, an anode current collector for an electrorefining cell, other electrically conducting components such as a cell-lining, and non-conductive cell components including separator walls, weirs and packing elements. These composite materials are formed by submitting particles of aluminum and the aluminum oxycompound and/or oxides which will form the aluminum oxycompound by reaction with the aluminum, and optionally with powders of the additives, to a heat treatment. Typically the particles are hot pressed or cold pressed and then heated. However, it is difficult by these methods to provide a highly dense structure and, furthermore, when a high aluminum content is desired to enhance the electrical conductivity it is difficult to obtain a stucture which remains rigid at the operating temperature (about 1000.degree. C.). Conversely, where conductivity is desired and aluminum is the sole conductive agent, it has been found that the aluminum should account for about 15-40 vol % of the material.